To provide a polysiloxane based composition excellent in molding processability, transparency, heat resistance, light resistance and so on, and reducing the occurrence of cracking during curing.

The polysiloxane based composition comprises, as essential ingredients, a modified polysiloxane having a polyhedral structure obtained by modification of a hydrosilyl- and/or alkenyl-containing compound, which can undergo a hydrosilylation reaction with (a) an alkenyl- and/or hydrosilyl-containing polysiloxane based compound of a polyhedral structure, and a non-linear polysiloxane having at least one hydrosilyl or alkenyl group.